This application claims the priority of Korean Patent Application No. 2003-14614, filed on Mar. 8, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser diode and more particularly, to a semiconductor laser diode having a ridge waveguide.
2. Description of the Related Art
As high density information recording is increasingly in demand, the need for a visible light semiconductor laser diode is increasing. Therefore, semiconductor laser diodes made of various compounds capable of emitting a visible light laser are being developed. In particular, much attention has been paid to a group III-V nitride semiconductor laser diode because its optical transition is a direct transition type that induces high frequency laser emission and because it emits a blue light laser.
FIG. 1 shows a perspective view of a conventional GaN-based, group III-V nitride semiconductor laser diode having n-type and p-type electrodes, which are formed on the same side, and a ridge waveguide.
Referring to FIG. 1, an n-type material layer 20, a light emitting active layer 30, and a p-type material layer 40 are sequentially formed on a sapphire substrate 10. The upper surface of the p-type material layer 40 is formed with a ridge waveguide 70. The ridge waveguide 70 is slightly protruded from the upper surface of the p-type material layer 40. The ridge waveguide 70 comprises a channel 71 formed so that the p-type material layer 40 is exposed in a narrow stripe-type configuration, and a p-type electrode layer 50, which is in contact with the p-type material layer 40 via the channel 71. Strictly speaking, a reference numeral 2 is not the p-type material layer 40 but is a current restricting layer formed for defining the channel 71.
An n-type electrode layer 60 serves to feed an electric current into a bottom material layer 21 of the n-type material layer 20 and is formed on an exposed surface 22 of the bottom material layer 21 of the n-type material layer 20.
In this structure, the upper surface of the p-type electrode layer 50, that is, the upper surface 72 of the ridge waveguide 70, and the upper surface of the n-type electrode layer 60 are separated by a step height, h1.
Generally, a temperature has an effect on a critical current and laser mode stability for laser emission of semiconductor laser diodes. As a temperature increases, both of the characteristics are lowered. Therefore, there is a need to remove heat generated in an active layer during laser emission to thereby prevent overheating of laser diodes. In the structure of the aforementioned conventional GaN-based, group III-V semiconductor laser diode, most heat is discharged only through a ridge because of very low thermal conductivity of a substrate (for a sapphire substrate, about 0.5 W/cmK). However, because heat discharge through a ridge occurs limitedly, it is difficult to carry out efficient heat discharge. Therefore, lowering of characteristics of semiconductor devices by overheating of laser diodes is not efficiently prevented.
In this regard, a flip-chip bonding technology shown in FIG. 2 can be applied to the structure of a conventional semiconductor laser diode shown in FIG. 1 to discharge heat generated in an active layer.
Referring to FIG. 2, a reference numeral 80 indicates a conventional GaN-based, group III-V semiconductor laser diode. A reference numeral 90 indicates a submount as a heat discharge structure, a reference numeral 91 a substrate, and reference numerals 92a and 92b first and second metal layers, respectively. Reference numerals 93a and 93b indicate first and second solder layers, which are respectively fused to an n-type electrode layer 60 and a p-type electrode layer 50 of the semiconductor laser diode 80.
By bonding the semiconductor laser diode to the submount, a separately prepared heat discharge structure, heat discharge efficiency can be increased.
However, as shown in FIG. 2, the first solder layer 93a is thicker than the second solder layer 93b by the height of h1 in order to compensate for the step height, h1 between the p-type electrode 50 and the n-type electrode 60. Due to such a thickness difference, the first and second solder layers 93a and 93b may not concurrently be molten.
The first and second solder layers 93a and 93b are generally made of a metal alloy, and thus, even if the chemical composition ratios of the first and second solder layers 93a and 93b slightly differ from each other, there is a large difference between their melting temperatures. In a case wherein the first and second solder layers 93a and 93b differ in thickness in a method of manufacturing the submount, the first and second solder layers 93a and 93b must be formed under separate two processes, not under a single process. As a result, there exists a likelihood for the first and second solder layers 93a and 93b to have different chemical composition ratios.
The ridge waveguide 70 is protruded from the p-type material layer 40, and although exaggerated in FIG. 2, has a width W1 of no more than several micrometers. Therefore, when the semiconductor laser diode 80 is bonded to the submount 90, a thermal stress may be concentrated on the ridge waveguide 70. In addition, when the first and second solder layers 93a and 93b are not concurrently fused as mentioned above, the submount 90 may be inclined to one side. In this case, a mechanical stress may be concentrated on the narrow ridge waveguide 70.
Stresses concentrated on the ridge waveguide 70 may affect light emission in the active layer 30 below the ridge waveguide 70.
FIG. 3 shows an image plane photograph of laser light emission taken along the longitudinal direction A of the stripe-like ridge waveguide 70. As shown in FIG. 3, light is emitted unevenly and discontinuously along the longitudinal direction A of the ridge waveguide 70.